Conventionally, in the manufacturing industry of a printed wiring board, a cover lay film has been used as a surface protection material for a flexible circuit substrate, which cover lay film is obtained by applying an adhesive to a shaped article such as polyimide film. In a case where this cover lay film is adhered to a flexible circuit substrate, it is a general practice to form, by punching etc., openings in the cover lay film beforehand so that the openings positionally correspond to places where terminals of circuits are to be located or where parts are to be connected and, after positioning the cover lay film, attach the cover lay film to the flexible circuit substrate by thermocompression bonding such as thermal pressing. Further, the cover lay film is required to have excellent characteristics such as heat resistance, chemical resistance, flame retardancy, adhesiveness, electrical insulation reliability and the like. As to flame retardancy, it is possible to obtain good flame retardancy by using a halogen flame retardant. However, in view of environmental loads, a flame-retarding technique involving no halogen flame retardant is required.
The cover lay film is poor in position accuracy and also low in workability of the attachment because forming an accurate opening is difficult and, further, positioning the cover lay film at a time of attachment is often performed manually. Therefore, a photosensitive solder resist is used in some cases. The use of photosensitive solder resist provides excellent microfabricability to the cover lay film. As the photosensitive solder resist, a photosensitive resin composition which is mainly made of an epoxy resin etc. is used. This photosensitive solder resist is excellent in electrical insulation reliability as an electrical insulation material. However, the photosensitive solder resist has disadvantages in mechanical characteristics such as flexibility, and greatly shrinks at curing. Accordingly, in a case where the photosensitive solder resist is layered on a circuit substrate which is thin and flexible (i.e. a flexible circuit substrate), the circuit substrate is greatly warped. Therefore, it has been difficult to use the photosensitive solder resist for a flexible circuit substrate. Further, the photosensitive solder resist is also poor in flame retardancy. In a case where a flame retardant is added to the photosensitive solder resist so as to increase the flame retardancy, there are problems such as a property deterioration and such as a contact fault and process pollution which occur due to a bleedout in which the flame retardant bleeds from a cured film.
In recent years, various suggestions are made on this photosensitive solder resist.
For example, Patent Literature 1 suggests a flame retardant photocurable resin composition containing a phosphazene compound which is in a liquid state at a room temperature, a carboxyl group containing resin, and a photopolymerization initiator. Further, Patent Literature 2 suggests a flame retardant photocurable resin composition which contains a carboxyl group containing resin, a soluble phosphazene compound which is soluble in a carboxyl group containing resin by 5 wt % or more, and a photopolymerization initiator.
Patent Literature 3 suggests a photocurable and thermosetting resin composition (i) which excels in tack-free dryness, adhesion, and resolution, (ii) which scarvely generates mist during thermal curing, and (iii) which is highly sensitive and excellent in storage stability.
Citation List
[Patent Literature 1]
Japanese Patent Application Publication, Tokukai, No. 2010-39389 A (Publication Date: Feb. 18, 2010)
[Patent Literature 2]
Japanese Patent Application Publication, Tokukai, No. 2010-113245 A (Publication Date: May 20, 2010)
[Patent Literature 3]
Pamphlet of International Application Publication, No. 2004/048434 (International Publication Date: Jun. 10, 2004)